Pulse-echo radio system testing means



S. l. TOURSHOU Filed June l2, 1944 PULSE-ECHO RADIO SYSTEM TESTING MEANS May 6, 1947;

| l l. N QWSSK Patented May 6, 1947 anni PULSE-ECHO RADIO SYSTEM TESTING MEANS Simeon I. Tourshou, Philadelphia, Pa., assigner to Radio Corporation of America, a corporation of Delaware Application June 12, 1944, Serial No. 539,917

6 Claims.

My invention relates to radio pulse-echo systems and particularly to` a system designed to give an indication whenever a radio wave reflecting object moves into a certain region or locaion.

The invention will be described particularly with respect to a warning system for an aircraft such as a fighter airplane. The system is designed especially to warn the pilot when anenemy airplane approaches from the rear and comes into a danger zone. To accomplish this, the pulse receiver isblocked except during the application of a gate pulse which is timed to allow only pulses reflected from the danger zone to pass through the receiver. If the pilot is to rely on the warning system, itis important that he have some way of checking the system periodically to make certain that it is in operative condition.

An object of the present invention is to provide a method of and means for checking a warning system of the above-mentioned typeV to determine whether it is in operative condition.

A further object of the invention is to .provide an improved pulse-.echo system wherein means is provided to check the system as to operativeness.

A still further object of the invention is to provide an improved radio pulse-echo warning system for fighter aircraft.

According to a preferred embodiment of the invention, the checking circuit. comprises an amplier tube, which Will be referred to as the checker tube, having in its grid circuit an inductance coill that is tuned to frequency that is much higher than the pulse Vrepetition rate. Some of the energy of a transmitted pulse feeds directly into the receiver, and this received pulse is applied to the tuned inductance coil whereby it rings and produces a damped wave upon the occurrence of each transmitted pulse. These damped waves last long enough to extend into the region of the gate pulse. If a checkingswitch is closed, the damped waves are amplified by the checker tube and applied to the receiver input circuit whereby an indicationL of pulse reception will be obtained if the system is in operative condition.

Otherf objects; features and advantages of the invention will appear from the ollowingdescrip.- tion taken in connection with the accompanying drawing in which the single gure is a circuit and block diagram of a preferred embodiment of the invention.

The drawing shows acomplete pulse-echosystem that is to be locatedV in Van airplane with a directive antenna l0 positioned to radiate pulses to the rear. A receiving antenna (not shown) is similarly positioned to receive these pulses after reection. Or, if preferred, a single antenna may be duplexed in a well known manner to function for both transmitting and receiving.

The radio pulses are supplied to the antenna l!) from a radio pulse transmitter H. The timing of the pulse modulation is controlled by a timing oscillator l2. A gate pulse forming circuit l3 which receives pulses from the transmitter Il supplies positive gate pulses I4 of the correct timing and duration to the screen grid I6 of one or more intermediate-frequency amplifier tubes Il. One specific circuit design for the transmitter and gate pulse circuit is described and claimed in copending application Serial No. 515,496, filed December 24, 1943, in the names of Simeon I. Tourshou and Robert Trachtenberg and entitled Radio pulse-echo system.

The pulse receiver portion of the system is of the superheterodyne type comprising a rst detector or converter 2| having a tuned input circuit or line 22- and further comprising a local oscillator 23 coupled to the converter 2l through a coupling loop 24. The receiving antenna (not shown) is coupled through a coupling loop 2S to the tuned input circuit 22. The receiving antenna may be tuned by a variable capacitor 21.

The converter 2l may be of the full-wave rectier type consisting of detector tubes 23 and 29. .The grids of tubes 28 and 29 are connected to opposite sides of the tuned line 22. A coil 31, which acts as a choke coil at the carrier` frequency, is connected across the tuned line 22 and grounded at its midpoint. The coil 31| has a low impedance at the intermediate frequency to avoid degeneration. A cathode bias resistor 32, shunted by a capacitor 331, is connected between the grounded midpoint and the cathodes of the tubes 2B and 29. The anodes of the tubes 28 and 29 arevconnected to the-primary coil 34 of an intermediate-frequency transformer 36.

TheA tunedV input circuit 22 may be tuned to the carrier frequency of the pulse transmitter Il by means of a shorting bar 31 and a variable capacitor 38.

The local-oscillator 23V may comprise a pair o vacuumv tubes 4I and 42 that are connected in parallel with each other. Their grids are connected to one side-of a tuned line 43 through a coupling capacitor 44 while their anodes are connected tothe other sideof the line 53 through a, coupling capacitor 46. The cathodes of tubes 4I and 42 are connected through an inductance coil 41 to ground, The oscillator 23 is tuned by means of a shorting bar 48 to the correct frequency for producing the desired intermediate frequency (30 megacycles in the present example). All tuning of the receiver is fixed after the initial tuning adjustments.

The grids of oscillator tubes 4| and 42 are also connected through a grid leak resistor 5I to ground. In parallel with resistor 5I there is a resistor 52 and a resistor 53 in series with each other. The resistor 53 is shunted by a capacitor 54 whereby a bias voltage for the oscillator 23 (and also for the checker tube 6I as explained hereinafter) builds up across resistor 53.

In operation, the I.F. amplier is blocked by suitable biasing on the amplifier tube I5 and on other I.F. amplier tubes (not shown) in the absence of a positive gate pulse I4. Therefore, a reflected pulse picked up by the receiving antenna can pass through the I.F. amplifier only if the pulse has been reected from an aircraft that has entered the danger zone corresponding to the timing and duration of the gate pulse. In that event, if the warning system is in operative condition, the reected pulses pass through the I.F. amplifier, are detected and amplied by a second detector and a video amplifier indicated at 55, and are applied to a relay and indicator unit 51 that warns the pilot that an enemy aircraft has entered the danger Zone.

Referring now to the checking circuit itself, it includes an amplifier tube 6I, referred to as the checker tube, and a tuned coil 62 connected between the control grid 63 of the tube 6I and the high potential end of the bias resistor 53.. The coil 62 may resonate with its distributed capacity, indicated by the dotted lines, for example. The coil 62 is tuned to 7% megacycles per second in the specific example being described whereby it rings when shock excited by a pulse 64, received directly from the transmitter, to produce a damped wave train. The pulses 54 for ringing the coil B2 are applied from the cathode of the converter tubes 28 and 29 through a blocking capacitor 66 and a conductor 61 to the grid end of the coil 62. The pulses 64 may recur at the rate of 400 per second and have a duration of 0.4 microsecond, for example.

It should be noted that the pulses 64 are video frequency pulses of large amplitude, their amplitude being of the order of 100 volts, for example. That they are video frequency pulses results from the fact that the pulses which are transmitted directly from the transmitter to the converter 2| are of such large amplitude compared with the amplitude of the local oscillator signal that the converter 2| recties them substantially independently of the local oscillator.

A checking switch 63 is provided which is normally open. When the switch 58 is closed to check the system for operativeness, it connects the cathode 59 of tube 6I to ground whereby voltage is applied between the cathode 69 and the anode and screen grid of the tube 6I so that the damped wave trains from the coil 62 are amplied and supplied through a blocking capacitor 1I to a second tuned coil 12 in the anode circuit of the tube 6I. The coil 12 may resonate with its distributed capacity, indicated by the dotted lines, for example. The coil 12 is tuned to double the frequency to which coil 52 is tuned, i. e., to 15 megacycles in the present example. Thus, there are trains of damped waves 13 appearing across the tuned coil 12. The damped wave trains 13 are applied from the anode end of coil 12 through a conductor 14 and a coupling capacitor 16 to the cathodes of the converter tubes 28 and 29. The 15 megacycle wave trains 13 generate in the plate circuit of converter 2| second harmonic wave trains of 30 megacycles,y

which are impressed upon the input circuit of the 30 megacycle I.F. amplifier. Since a wave train 13, and the corresponding second harmonic wave train, have not yet died down when the gate pulse I4 occurs, a portion of the second harmonic wave train will pass through the I.F. amplifier and operate the indicator unit 51 if the complete pulse-echo system is operative. The use of the 30 megacycle second harmonic output of the converter 2| is advantageous both because it simplifies the filtering required for preventing feedback and because it makes it easier to produce wave trains of the desired amplitude at the ringing coil 12.

Referring againV to the checking tube 6I and associated connections, a D.C. operating voltage is applied to its anode and screen grid through resistors 11 and 18, respectively. The cathode 59 preferably is connected to ground for alternating current through a capacitor 19.

It may be noted that the coil 12 may be omitted if the pulse 64 has a suiciently steep Wave front. In the present example, however, where the comparatively wide pulse 54 of 0.4 microsecond is employed, it is desirable to shock excite a 15 megacycle tuned coil by means of the intermediate circuit comprising the 7.5 megacycle tuned coil B2.

Since the I.F. amplifier has a band width of 4 to 5 megacycles, the coils 62 and 12 need not be tuned exactly to '1.5 megacycles and 15 megacycles, respectively, although the one-to-two tuning ratio should be maintained so that successive cycles of the wave trains across coil 62 will reinforce the ringing of coil 12. For example, the coils 62 and 12 may be tuned to 7 and 14 megacycles, respectively, or to 6.75 and 13.5 megacycles, respectively.

It may be noted also that the checker tube 6I may be replaced by a relay providing it is not required that the circuit shall check the operation of the local oscillator. In this case, the wave trains from the coil 62 may be applied directly to the coil 'I2 (or through a tube functioning only as an amplier) while the relay normally maintains a short circuit across the coil 12. To check the apparatus, the relay is actuated to remove the short circuit from the coil 12 whereby the wave trains 13 are applied to the converter 2|.

From the foregoing description it will be evident that most of the system components are checked as to operativeness when the checking switch 68 is closed. For example, the indicator will not be actuated under the following conditions:

(1) If there are no pulses being transmitted or if the transmitted pulses are of weak signal strength.

(2) If the gate pulse I4 is not being generated.

(3) If either the converter 2|, the I.-F. amplifier, the second detector or the video amplifier is inoperative.

(4) If the oscillator 23 is not oscillating, since this results in taking bias off the grid 63 of the checker tube 6I.

I claim as my invention:

1. In a pulse-echo system comprising a pulse transmitter and a pulse receiver and further comprising gating means for causing the receiver to pass a received pulse during a, predetermined time interval, a checking circuit for checking the operativeness of said system, said checking circuit comprising a circuit th'at is resonant at a fre-- quency which is high with respect to the repetition rate of the transmitted pulses, means for applying to said resonant circuit pulses which are received directly from the transmitter whereby each of said received pulses produces a damped Wave train, and means for applying said wave trains to the input circuit of the pulse receiver, each of said wave trains extending into said predetermined time interval whereby the receiver may pass a portion of each wave train.

2. In a pulse-echo system comprising a pulse transmitter and a pulse receiver and comprising gating means for causing th'e receiver to pass a received pulse during a predetermined time interva1,.said receiver being of the superheterodyne type comprising a rst detector and a local oscillator, a checking circuit for checking operativeness of said system, said checking circuit comprising an amplifier tube and an associated tuned circuit that is resonant at a frequency which is high with respect to the repetition rate of the transmitted pulses, means for shock exciting said tuned circuit by means of pulses received directly from the transmitter whereby each of said received pulses produces a damped wave train, said last-mentioned wave trains extending into said predetermined time interval, and means for applying said last-mentioned wave trains to the input circuit of said iirst detector.

3. In a pulse-echo system comprising a Pulse transmitter and a pulse receiver and comprising gating means for causing the receiver to pass a received pulse during a predetermined time interval, said receiver being of the superheterodyne type comprising a converter and a self-biased local oscillator, a checking circuit for checking operativeness of said system, said checking circuit comprising an amplifier tube to which at least a portion of said oscillator self-bias is applied and an associated tuned circuit that is resonant at a frequency which is high with respect to the repetition rate of the transmitted pulses, means for shock exciting said tuned circuit by means of pulses received directly from the transmitter whereby each of said received pulses produces a damped wave train, said last-mentioned wave trains extending into said predetermined time interval, and means for applying said lastmentioned wave trains to the input circuit of the pulse receiver.

4. In a pulse-echo system comprising a pulse transmitter and a pulse receiver and further comprising gating means for causing the receiver to pass a received pulse during a predetermined time interval, a checking circuit for checking the operativeness of said system, said checking circuit comprising an amplifier tube having an input circuit and an output circuit, said input circuit being resonant at a frequency that is high with respect to the repetition rate of the transmitted pulses, means for applying pulses received directly from the transmitter to said resonant input circuit whereby each' of said received pulses produces a damped wave train, a resonant circuit in the output circuit of said tube which is tuned to a harmonic of the frequency to which said input circuit is tuned whereby damped wave trains appear across said output circuit, and means for applying said last-mentioned wave trains to the input circuit of the pulse receiver, each of said last-mentioned wave trains extending into said predetermined time interval whereby the receiver may pass a portion of each Wave train.

5. In a pulse-echo system comprising a pulse transmitter and a pulse receiver and comprising gating means for causing the receiver to pass a received pulse during a predetermined time interval, said receiver comprising a detector and an amplifier, a checking circuit for checking operativeness of said system, said checking circuit comprising a tuned circuit that is resonant at a frequency that is equal to one-half one of the frequencies falling within the pass range of said amplifier, said resonant frequency being high with respect to the repetition rate of the transmitted pulses, means for shock exciting said tuned circuit by means of pulses received directly from the transmitter whereby each of said received pulses produces a damped wave train, said wave trains extending into said predetermined time interval, means for applying said wave trains to said detector with said wave trains h'aving sufficient amplitude to produce a second-harmonic wave train, and means for applying said secondharmonic wave train to said amplifier.

6. In a pulse-echo system comprising a pulse transmitter and a pulse receiver and comprising gating means for causing the receiver to pass a received pulse during a predetermined time interval, said receiver being of the superheterodyne type comprising a iirst detector, a local oscillator, and an intermediate-frequency amplier, a checking circuit for checking operativeness of said system, said checking circuit comprising an amplier tube and an associated tuned circuit that is resonant at a frequency that is equal to substantially onehalf the frequency to which said intermediatefrequency amplifier is tuned, said resonant frequency being high with' respect to the repetition rate of the transmitted pulses, means for shock exciting said tuned circuit by means of pulses received directly from the transmitter whereby each of said received pulses produces a damped wave train, said wave trains extending into said predetermined time interval, means for applying said wave trains to said first detector with said wave trains having sufficient amplitude to produce a second-harmonic wave train, and means for applying said second-harmonic wave train to said intermediate-frequency amplifier.

SIMEON I. TOURSHOU. 

